Takeshi Asai

The biomechanics of kicking in soccer: A review

Abstract Kicking is the defining action of soccer, so it is appropriate to review the scientific work that provides a basis of our understanding of this skill. The focus of this review is biomechanical in nature and builds on and extends previous reviews and overviews. While much is known about the biomechanics of the kicking leg, there are several other aspects of the kick that have been the subject of recent exploration. Researchers have widened their interest to consider the kick beginning from the way a player approaches the ball to the end of ball flight, the point that determines the success of the kick. This interest has encapsulated characteristics of overall technique and the influences of the upper body, support leg and pelvis on the kicking action, foot–ball impact and the influences of footwear and soccer balls, ball launch characteristics and corresponding flight of the ball. This review evaluates these and attempts to provide direction for future research.

Fundamental aerodynamics of the soccer ball

When the boundary layer of a sports ball undergoes the transition from laminar to turbulent flow, a drag crisis occurs whereby the drag coefficient (Cd) rapidly decreases. However, the aerodynamic properties and boundary-layer dynamics of a soccer ball are not yet well understood. In this study we showed that the critical Reynolds number (Recrit) of soccer balls ranged from 2.2 × 105 to 3.0 × 105. Wind-tunnel testing, along with visualisation of the dynamics of the boundary layer and the trailing vortex of a ball in flight, demonstrated that both non-spinning and spinning (curved) balls had lowCd values in the super-critical region. In addition, theRecrit values of the soccer balls were lower than those of smooth spheres, ranging from ∼ 3.5 × 105 to 4.0 × 105, due to the effects of their panels. This indicated that the aerodynamic properties of a soccer ball were intermediate between those of a smooth ball and a golf ball. In a flow visualisation experiment, the separation point retreated and theCd decreased in a super-critical regime compared with those in a sub-critical regime, suggesting a phenomenon similar to that observed in other sports balls. With some non-spinning and spinning soccer balls, the wake varied over time. In general, the high-frequency component of an eddy dissipated, while the low-frequency component increased as the downstream vortex increased. The causes of the large-scale fluctuations in the vortex observed in the present study were unclear; however, it is possible that a ‘knuckle-ball effect’ of the non-rotating ball played a role in this phenomenon.

Effect of panel shape of soccer ball on its flight characteristics

Soccer balls are typically constructed from 32 pentagonal and hexagonal panels. Recently, however, newer balls named Cafusa, Teamgeist 2, and Jabulani were respectively produced from 32, 14, and 8 panels with shapes and designs dramatically different from those of conventional balls. The newest type of ball, named Brazuca, was produced from six panels and will be used in the 2014 FIFA World Cup in Brazil. There have, however, been few studies on the aerodynamic properties of balls constructed from different numbers and shapes of panels. Hence, we used wind tunnel tests and a kick-robot to examine the relationship between the panel shape and orientation of modern soccer balls and their aerodynamic and flight characteristics. We observed a correlation between the wind tunnel test results and the actual ball trajectories, and also clarified how the panel characteristics affected the flight of the ball, which enabled prediction of the trajectory.

A comparison of Jabulani and Brazuca non-spin aerodynamics

Wind-tunnel experimental measurements of drag coefficients for non-spinning Jabulani and Brazuca balls are presented. The Brazuca ball’s critical drag speed is lower than that of the Jabulani ball, and the Brazuca ball’s super-critical drag coefficient is larger than that of the Jabulani ball. Compared to the Jabulani ball, the Brazuca ball suffers less instability due to knuckle-ball effects. Using drag data, numerically determined ball trajectories are created, and it is postulated that although power shots are too similar to note flight differences, goalkeepers are likely to note the differences between Jabulani and Brazuca ball trajectories for intermediate-speed ranges. This latter result may appear in the 2014 World Cup for goalkeepers used to the flight of the ball used in the 2010 World Cup.

Aerodynamic drag of modern soccer balls

Soccer balls such as the Adidas Roteiro that have been used in soccer tournaments thus far had 32 pentagonal and hexagonal panels. Recently, the Adidas Teamgeist II and Adidas Jabulani, respectively having 14 and 8 panels, have been used at tournaments; the aerodynamic characteristics of these balls have not yet been verified. Now, the Adidas Tango 12, having 32 panels, has been developed for use at tournaments; therefore, it is necessary to understand its aerodynamic characteristics. Through a wind tunnel test and ball trajectory simulations, this study shows that the aerodynamic resistance of the new 32-panel soccer ball is larger in the high-speed region and lower in the middle-speed region than that of the previous 14- and 8-panel balls. The critical Reynolds number of the Roteiro, Teamgeist II, Jabulani, and Tango 12 was ~2.2 × 105 (drag coefficient, Cd ≈ 0.12), ~2.8 × 105 (Cd ≈ 0.13), ~3.3 × 105 (Cd ≈ 0.13), and ~2.4 × 105 (Cd ≈ 0.15), respectively. The flight trajectory simulation suggested that the Tango 12, one of the newest soccer balls, has less air resistance in the medium-speed region than the Jabulani and can thus easily acquire large initial velocity in this region. It is considered that the critical Reynolds number of a soccer ball, as considered within the scope of this experiment, depends on the extended total distance of the panel bonds rather than the small designs on the panel surfaces.

Flow Visualization on a Real Flight Non-spinning and Spinning Soccer Ball

Wind tunnel test with a full size non-spinning soccer ball was carried out to measure the aerodynamic forces (drag, lift and side force). The drag coefficient of a non-spinning soccer ball in the subcritical regime was nearly 0.43 and that of in the supercritical regime was nearly 0.15. It is estimated that the critical Reynolds number of a soccer ball is about 2.2×105. In the visualization experiment using titanium tetrachloride, we compared the flow around the ball during a non-spinning, low-speed kick (5 m/s) and a high-speed kick (29 m/s). During the low-speed kick, the boundary layer separation point was about 90 degrees from the front stagnation point, while during the high-speed kick the separation point had receded to about 120 degrees from the front stagnation point.

Comparison of Kicking Motion Characteristics at Ball Impact between Female and Male Soccer Players

Kicking, and particularly foot–ball impact, is a fundamental soccer skill. To date, most studies of ball impact have examined only male players. However, gender differences among soccer players might affect kicking performance. The competitiveness of female players might therefore be enhanced by providing gender-specific training and coaching. Here we compared the technical characteristics of ball impact during instep and inside kicks between male and female players. Female players exhibited significantly lower ball velocity, foot velocity immediately before impact, striking mass, and average ball-to-foot velocity ratio than male players (p < 0.05). The ball-to-foot velocity ratio decreased as the distance of the impact point from the centre of gravity (CoG) of the foot increased. Our results suggested that training for female players should focus on making contact with the ball near the foots CoG under various conditions.

A Study of Knuckling Effect of Soccer Ball (P106)

The aerodynamic properties and boundary-layer dynamics of a non-spinning or slowly-spinning soccer ball are not well understood. The purpose of this study is to discuss the magnitude and the frequency of the side force of non-spinning or slowly-spinning flight soccer ball, which called “knuckling effect ball”, using a high speed VTR image of a real place kick. The direct liner transformation method was used to obtain three dimensional coordinates of ball position. The magnitude and the frequency of the side force were measured by a digitizing software system in PC. The magnitude of the side force in real flight was measured to range from about 1 N to 8 N. Additionally, the frequency of the side force in real flight was estimated to range from 1.0 Hz to 3 Hz.

Biomechanics and Aerodynamics in Soccer

Most of the physical activity carried out in soccer consists of either primary movements (e.g. sprinting, jumping etc.) or kicking a football (with the aim being either an accurate pass to another player or the scoring of a goal). This chapter describes a number of studies that have been carried out to examine the physical interactions that occur during these situations.

Visualization of air flow around soccer ball using a particle image velocimetry.

A traditional soccer ball is constructed using 32 pentagonal and hexagonal panels. In recent years, however, the likes of the Teamgeist and Jabulani balls, constructed from 14 and 8 panels, respectively, have entered the field, marking a significant departure from conventionality in terms of shape and design. Moreover, the recently introduced Brazuca ball features a new 6-panel design and has already been adopted by many soccer leagues. However, the shapes of the constituent panels of these balls differ substantially from those of conventional balls. Therefore, this study set out to investigate the flight and aerodynamic characteristics of different orientations of the soccer ball, which is constructed from panels of different shapes. A wind tunnel test showed substantial differences in the aerodynamic forces acting on the ball, depending on its orientation. Substantial differences were also observed in the aerodynamic forces acting on the ball in different directions, corresponding to its orientation and rotation. Moreover, two-dimensional particle image velocimetry (2D-PIV) measurements showed that the boundary separation varies depending on the orientation of the ball. Based on these results, we can conclude that the shape of the panels of a soccer ball substantially affects its flight trajectory.